Recently, with the widespread use of personal computers, word processors and other apparatuses, liquid crystal displays have been widely adopted as their display devices instead of power-consuming, large-size CRTs, because they are thin and light and because they are also driven by batteries.
Driving methods for liquid crystal displays are generally classified into two types, the simple-matrix type and the active-matrix type. It is comparatively difficult to produce liquid crystal displays of the active-matrix type because non-linear elements are required for respective pixels that are arranged in the form of a matrix. Therefore, at present, those of the simple-matrix type are generally used for liquid crystal displays with large display capacities.
In the liquid crystal displays of the simple-matrix type, however, irregularity in display (crosstalk), which is dependent on display patterns, tends to occur as their display capacity increases due to their inherent characteristic, and this tends to cause degradation in display quality.
The following description will discuss the irregularity in display by using as an example a conventional liquid crystal display of the simple-matrix type using the averaged voltage method.
As illustrated in FIG. 13, the liquid crystal display is provided with: a liquid crystal panel 101 having a plurality of scanning electrodes Y1 through Y8 and a plurality of signal electrodes X1 through X8 which are arranged in a manner orthogonally intersecting one another; a scanning-side driving circuit 103 for successively applying voltages to the scanning electrodes Y1 through Y8; a signal-side driving circuit 102 for applying signal voltages representative of display data to the signal electrodes X1 through X8; a power-source circuit 104 for generating voltages required for the driving operation; and a control circuit 105 for controlling the scanning-side driving circuit 103 and the signal-side driving circuit 102.
The scanning electrodes Y1 through Y8 are successively scanned, and when selected, they are subjected to selected voltages applied from the power source circuit 104, and when not selected, they are subjected to non-selected voltages applied from the power source circuit 104. The signal electrodes X1 through X8 are, on the other hand, subjected to an on-voltage or an off-voltage that is applied from the power source circuit 104 depending on display data so as to drive the electrodes. Here, for convenience of explanation, it is assumed in the following explanation that eight scanning electrodes and eight signal electrodes are respectively provided, that these electrodes are driven by one eighth duty, and that the signal-reversing cycle for ac-based drive is three scanning lines.
The power source circuit 104 generates not only driving voltages V2 and V4 to be applied to the signal-side driving circuit 102, but also driving voltages V1, V3 and V5 to be applied to the scanning-side driving circuit 103. Further, the control circuit 105 releases to the signal-side driving circuit 102, a display data D, a data-shift clock CK, a scanning clock LP, a scanning start signal FLM, and an ac-conversion signal FR, as well as releasing to the scanning-side driving circuit 103, a scanning clock LP, a scanning start signal FLM, and an ac-conversion signal FR.
Referring to timing charts shown in FIGS. 14(a) through 14(g), the following description will discuss the operations of the respective driving circuits in the liquid crystal display having the above-mentioned arrangement.
FIGS. 14(a) through 14(g) show optimal waveforms of voltages that are applied to respective pixel A (an intersection of signal electrode X2 and scanning electrode Y2), pixel B (an intersection of signal electrode X3 and scanning electrode Y2) and pixel C (an intersection of signal electrode X6 and scanning electrode Y2) on the liquid crystal panel shown in FIG. 13. Here, on the liquid crystal panel of FIG. 13, pixels indicated by white circles are on-state elements, and pixels indicated by black circles are off-state elements. In the optimal stage, equal effective voltages are respectively applied to pixels A, B and C, and it is supposed that no variations occur in the transmittance of the liquid crystal display panel.
However, in an actual operation in a liquid crystal panel, irregularity in display (crosstalk) tends to occur as described below. Let us suppose that longitudinal black block displays in a while background or alternative black and white displays in a stripe are made as shown in FIG. 13. Pixels located on the same signal line as the longitudinal block portion (for example, pixel B) become brighter than the other background portion (for example, pixel A) (as indicated by slanting lines in FIG. 13), while pixels located on the same signal line as the alternative black and white stripe portion(for example, pixel C) become darker than the other background portion (for example, pixel A) (as shown by cross-hatching in FIG. 13).
The crosstalk of this type, which causes serious degradation in display quality, forms a major problem to be solved in the liquid crystal displays of the simple-matrix type. The following description will discuss causes of the crosstalk; however, the crosstalk which appears in the block portion and that which appears in the alternative black and white stripe portion differ from each other in their causes. Therefore, for convenience of explanation, the former is referred to as crosstalk of A type, and the latter is referred to as crosstalk of B type.
First, an explanation will be given on the cause of the crosstalk of A type.
FIGS. 15(a) and 15(b) show modified models wherein Y2 line is simplified and each liquid crystal pixel, which is a capacitive element, is substituted by a capacitor. Here, a resistor 112 shows an output ON resistor of the scanning-side driving circuit 103 which is normally constituted of ICs. A resistor 113 shows an equivalent resistor of the scanning electrode Y2 which is normally constituted of transparent electrodes such as ITOs.
As shown in FIG. 15(a), upon the rising of the ac-conversion signal FR, a downward waveform distortion occurs in Y2 line due to a transient response, since the majority of the signal electrodes except X2 and so on are subjected to a switchover from voltage V2 to voltage V4. In contrast, as shown in FIG. 15(b), upon the falling of the ac-conversion signal FR, an upward waveform distortion occurs in Y2 line due to a transient response, since the majority of the signal electrodes except X2 and so on are subjected to a switchover from voltage V4 to voltage V2.
When waveform distortions on the scanning electrode side are reviewed by applying this model to FIGS. 14(a) through 14(g), waveforms as shown in FIGS. 16(a) through 16(g) are obtained. In other words, due to voltage inversions of the background portion such as X2, waveform distortions having directions as shown in FIG. 16(e) occur in the scanning electrodes of Y2 line.
A voltage having a waveform shown in FIG. 16(f) is applied to pixel A. A voltage having a waveform shown in FIG. 16(g) is applied to pixel B. As clearly shown by FIGS. 16(f) and 16(g), the effective value of the voltage to be applied to pixel A decreases, while the effective value of the voltage to be applied to pixel B increases. As a result, pixel B becomes brighter than pixel A, thereby causing crosstalk of A type. Additionally, FIG. 16(a) shows a waveform of the scanning clock LP, and FIG. 16(b) shows a waveform of the ac-conversion signal FR.
Next, an explanation will be given on the cause of the crosstalk of B type.
FIGS. 17(c) and 17(d) respectively show optimal binary-voltage waveforms that are applied to signal electrodes X2 and X6, and there is no difference recognized between signal electrode X2 and signal electrode X6 even when their effective values are compared. In an actual liquid crystal display, however, dull waveforms as shown in FIGS. 17(e) and 17(f) are applied due to internal resistance of the signal-side driving circuit 102 and resistive components of the signal electrodes inside the liquid crystal panel. Additionally, in FIGS. 17(e) and 17(f), these dull portions are indicated by straight lines in a simplified form for convenience of explanation; however, in an actual operation, they vary in response to charging and discharging waveforms applied to the capacity.
Therefore, as clearly shown by FIGS. 17(e) and 17(f), in signal electrode X6 which is subjected to more number of changes in the binary-voltage waveform to be applied to the signal electrode due to the stripe-shaped displays, the dull portions appear more often than those in signal electrode X2, and the effective value drops by the corresponding degree. For this reason, a pixel on signal electrode X6 (for example, pixel C) becomes darker than a pixel on signal electrode X2 (for example, pixel A), thereby causing crosstalk of B type.
In order to solve the above-mentioned problems, it has been proposed to reduce crosstalk of A type by applying a compensating voltage whose polarity is inverted to that of the distortion occurring in the scanning electrode (which is referred to as the first prior art described in, for example, Japanese Laid-Open Patent Publication No. 171718/1990 (Tokukaihei 2-171718), Japanese Laid-Open Patent Publication No. 348385/1992 (Tokukaihei 4-348385), Japanese Laid-Open Patent Publication No. 12030/1994 (Tokukaihei 6-12030), etc.)
Moreover, it has also been proposed to reduce crosstalk of B type by providing an inversion period which corresponds to a certain period in the driving period of one scanning line (which is referred to as the second prior art described in, for example, Japanese Laid-Open Patent Publication No. 333315/1993 (Tokukaihei 5-333315), Japanese Laid-Open Patent Publication No. 276794/1992 (Tokukaihei 4-276794), Japanese Laid-Open Patent Publication No. 130797/1991 (Tokukaihei 3-130797) (U.S. Pat. No. 5,400,049), etc.)
However, the above-mentioned first and second prior arts have the following problems.
In the first prior art, although there is a certain degree of reducing effect on the distortion occurring in the scanning electrodes, it is not possible to reduce crosstalk of B type caused by the distortion occurring in the signal electrodes and frequency characteristics.
In this case, it is necessary to provide a detection circuit for detecting distortion on the scanning side and a compensation circuit for compensating for the distortion. However, in an actual operation, time lag always occurs between the detection circuit and the compensation circuit, and it is therefore inevitable to have a certain degree of distortion in the scanning electrodes which is left at its rising portion. As a result, distorted waveforms, which still remain in spite of the compensation, are applied to the liquid crystal panel irregularly with low frequencies depending on display patterns.
In the case of large-size liquid crystal panels which are driven by using high duty ratios and liquid crystal panels which use liquid crystal materials of low-threshold-value voltage in order to achieve low costs, or in the case where liquid crystal panels are operated at high temperature, their frequency characteristics tend to deteriorate and variations in transmittance with respect to the frequency become greater. In such cases, the above-mentioned distorted waveforms, which still remain slightly, vary with low frequencies that are irregular and highly discernible by the eye; this causes problems such as flickers and conspicuous beats.
The second prior art makes it possible to equalize dull waveforms on the scanning electrode side to a certain degree and to reduce crosstalk of B type. However, since it needs to increase the number of inversions in voltage waveforms to be applied to the signal electrodes, crosstalk of A type is, on the contrary, increased by the corresponding degree.